The invention relates to an apparatus for magnetically recording and reading wide-band signals on a magnetic tape in signal tracks which extend obliquely of the tape adjacent each other; and more particularly, to such recorders having: an at least partly cylindrical drive spindle with a free end, which spindle is rotatable about an axis of rotation; at least one magnetic head which is rotatable in a circular path about an axis of rotation for recording and/or reading the oblique signal tracks on the magnetic tape; a head-drum system, which is detachably secured to a cylindrical portion of the drive spindle near its free end, and which has a first side which is situated adjacent said free end and a second side which is remote from said free end, which head-drum system carries the magnetic heads and is formed with a central bore for mounting the head-drum system on the drive spindle; and a centering device on the first side of the head-drum system for centering the head drum system on the drive spindle and comprising an elastically deformable wall portion on the first side of the head drum system, which wall portion cooperates with the drive spindle.
Such apparatus for magnetically recording and reading video signals, for a so-called helical-scan video recorder, is known from Austrian patent specification AT-PS 345577. In this known apparatus the head-drum system comprises a cylindrical head-drum which constitutes the rotary part of a drum unit for guiding the magnetic tape past the magnetic heads in a helical path, and a stationary drum which is coaxial with the rotary drum and is spaced therefrom by a narrow gap. On the stationary drum a helical guide edge for the magnetic tape is provided, extending over part of its circumference. As a result of its rotation the rotary head-drum locally produces a film of air between the magnetic tape and the drum unit. Thus, the friction between the magnetic tape and the drum unit is reduced. On its first side which is remote from the stationary drum the head-drum is provided with a clamping sleeve with a cylindrical outer surface, which constitutes the elastically deformable wall portion. Around this clamping sleeve a clamping ring is mounted, which is formed with a slot in its ring wall, so that the parts of the clamping ring which are spaced by the slot can be moved resiliently towards each other. On the one side of the slot a threaded bolt is fitted into the clamping ring and extends with clearance through the clamping ring on the other side of the slot. By tightening the bolt the clamping ring is deformed, so that the clamping sleeve of the head drum is also deformed and the head drum is centered and also clamped on the driving spindle.
In video recorders and in particular in cassette recorders intended for home-entertainment use a most economic use of the magnetic tape is pursued and therefore it is attempted to provide the magnetic tape with as dense a pattern of adjacent signal tracks as possible. Modern helical-scan video cassette recorders may employ slightly overlapping signal tracks having a width of approximately 40 microns and having a center-to-center spacing of the tracks of approximately 30 microns. The track length is approximately 100 mm. The tracks should be situated within narrow tolerances over the entire length, in particular because great value is attached to the possibility of playing back on one video recorder video programs recorded with another similar video recorder, without an appreciable loss of quality. A really successful interchangeability of cassettes provided with recorded programs and video recorders belonging to a standard system for recording/reproducing video signals is obtained only if a program recorded on one video cassette recorder can be played back on another video cassette recorder belonging to the same video-recording system without a perceptible loss of quality of the signal reproduced. This requirement is referred to as the compatibility of video recorders and the associated magnetic tape cassettes, which together belong to a standard system for recording and reproducing video signals. As a result of this only very small tolerance ranges are available for the position of the magnetic heads relative to the stationary tape-guide edge. In helical-scan video cassette recorders equipped with a plurality of magnetic heads such as video cassette recorders adapted for domestic use, it is essential that the heads are also accurately positioned relative to each other in view of the synchronism of the video signals which are recorded or head by the heads. Also in this case only small tolerance ranges are available because otherwise visible time tolerance ranges must be guaranteed. Yet it is desirable that these centering and mounting operations can be carried out by a service engineer rapidly and without the use of special alignment devices.
The central bore of this known head-drum has a certain length. The diameter of the bore is slightly greater than the diameter of the drive spindle. Owing to the small radial clearance which exists between the drive spindle and the wall of the central bore, the head-drum can readily be slid over the drive spindle. After the head-drum system has been slid onto the spindle it is secured to the drive spindle with the aid of the clamping device, the clearance between the wall of the central bore and the drive-spindle being locally eliminated at the location of the clamping device by the elastic deformation of the clamping sleeve. During clamping a certain tilting of the head-drum system relative to its axis of rotation is inevitable. Due to the clamping forces which are exerted, the clearance between the drive spindle and the wall of the bore on the second side of the head-drum system is eliminated completely on one side of 5 microns, in such a way that the mutual difference in height between the magnetic heads which should not exceed 5 microns. For a head-drum system with a rotary head-drum having a diameter of 65 mm and carrying two magnetic heads the eccentricity of the magnetic heads relative to the axis of rotation of the head drum in a typical modern video cassette recorder should be less than 11/2 micron. A further requirement which is imposed is that the magnetic heads are uniformly spaced over the circumference of the head-drum system in an accurate manner. For example, when two magnetic heads are used, the lengths of arc between the two magnetic heads should not differ more than 3 microns from each other.
The construction of the head-drum system should be such that mounting and also removing the head drum system during the manufacture of video cassette recorders, and for any subsequent servicing operations, can be effected rapidly and by the use of simple means. It is then a requirement that after removal of a head-drum system and the subsequent mounting of a new head-drum system, for example for servicing purposes, the desired compatibility is still quaranteed. In view of the very small tolerances which are permissible in respect of the eccentricity and the height position of the magnetic heads, the precise centering and the precise coaxial mounting of the head-drum system within these errors will appear in the reproduced video picture, i.e. instabilities will occur near the edge of the picture.
In view of the stringent accuracy requirements imposed on the positions of the magnetic heads use is generally made of special optical precision instruments for mounting the magnetic heads on the head-drum system. In the case of the head-drum systems for apparatus of the previously mentioned known type the head-drum system is intended to be first centered and mounted on a special mounting spindle with the aid of its own clamping device. After mounting and alignment of the magnetic heads the head-drum system is removed from the mounting spindle by releasing the clamping device, after which the head-drum system may be mounted on the drive spindle of a video cassette recorder. Obviously it is then essential that the adjustments performed with the aid of said mounting spindle are most accurately reproduced after the head-drum system has been mounted on the drive spindle of the video cassette recorder. In a modern helical scan video cassette recorder the height tolerance of the magnetic heads relative to the tape-guide edge should be adjusted with a tolerance of the order of magnitude of the bore. This tilting is undesirable because it contributes to the eccentricity of the magnetic heads and to errors in respect of the position of the magnetic heads relative to the supporting edge. The location where the drive spindle makes contact with the bore on the second side of the head-drum system is unpredictable and not reproducible. It is therefore highly probable that the direction of the tilting to which the head-drum system is subject during mounting of the said special mounting spindle will thus be different from the direction of the tilting to which the head-drum system is subject during mounting on the drive spindle of the video cassette recorder.